As is well known, the traveling-wire EDM process makes use of a thin metallic wire composed, say, of copper or brass and having a diameter, say, of 0.1 to 0.5 mm. The wire may be continuously unwound from a supply reel and passed through a workpiece and taken up onto a takeup roller. In the path of wire travel, a pair of machining positioning guide members are arranged to support and guide the traveling wire in machining relationship with the workpiece. A machining fluid, typically distilled water, is supplied into the cutting zone from nozzle means which preferably comprises two nozzles disposed on the opposite sides of the workpiece, respectively. Preferably, the machining fluid is injected into the cutting zone from a nozzle coaxially with the traveling wire electrode. An EDM power supply is electrically connected to the wire electrode and the workpiece to apply a machining current, commonly in the form of a succession of electrical pulses, therebetween. Time-spaced, discrete electrical discharges are thereby created across the machining gap defined between the traveling wire electrode and the workpiece to electroerosively remove material from the workpiece. As electroerosive material removal proceeds, the workpiece is displaced relative to the axis of the traveling wire electrode transversely thereto along a predetermined path which determines a contour of cut eventually imparted to the workpiece.
During the traveling-wire EDM process, the machining liquid tends to be heated up by successive electrical discharges. It has now been found that a rise in temperature of the machining liquid, especially when constituted by a distilled water liquid, is a source of reduction in the cutting accuracy, insufficiency of the removal rate and wire breakage. As the water liquid is heated up, its specific resistivity lowers and deviates from the desired setting, resulting in an enlargement of the machining gap size and the consequent deviation of the overcut. Furthermore, the cooling capacity of the water liquid when heated is reduced and the consequent increase in liability of the wire to break requires that the machining current be limited to an unsatisfactory level.